Quadrivalent meningococcal (MenACWY-TT) conjugate vaccine or a fourth dose of H. influenzae-N. meningitidis C/Y conjugate vaccine (HibMenCY-TT) is immunogenic in toddlers who previously received three doses of HibMenCY-TT in infancy.

•MenACWY-TT (+/− DTaP) was investigated in HibMenCY-TT-primed toddlers.•Prior to booster, 89.9%/96.3% of HibMenCY-primed toddlers had hSBA ≥1:8 for C/Y.•Post-booster 100% of toddlers had hSBA ≥1:8 for MenC/Y and ≥96.1% for MenA/W.•3 HibMenCY-TT doses prime for MenC/Y with persistence until the scheduled booster.•HibMenCY-TT-primed toddlers could be boosted with MenACWY-TT.